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The Unix and GNU/Linux command line

The Unix and GNU/Linux

command line

Michael OpdenackerThomas Petazzoni

Free Electrons

Abridged for ELE209 Lab 3 By Tim Toolan

© Copyright 2009, Free Electrons.Creative Commons BYSA 3.0 licenseLatest update: Feb 18, 2013, Document sources, updates and translations:http://freeelectrons.com/docs/commandlineCorrections, suggestions, contributions and translations are welcome!

http://free-electrons.com/docs/command-line


Displaying file contents

Several ways of displaying the contents of files.

cat file1 file2 file3 ... (concatenate)Concatenates and outputs the contents of the given files.

more file1 file2 file3 ...After each page, asks the user to hit a key to continue.Can also jump to the first occurrence of a keyword(/ command).

less file1 file2 file3 ...Does more than more with less.Doesn't read the whole file before starting.Supports backward movement in the file (? command).


The head and tail commands

head [<n>] <file>Displays the first <n> lines (or 10 by default) of the given file.Doesn't have to open the whole file to do this!

tail [<n>] <file>Displays the last <n> lines (or 10 by default) of the given file.No need to load the whole file in RAM! Very useful for huge files.

tail f <file> (follow)Displays the last 10 lines of the given file and continues to display new lines when they are appended to the file.Very useful to follow the changes in a log file, for example.

Exampleshead windows_bugs.txttail f outlook_vulnerabilities.txt


The grep command

grep <pattern> <files>Scans the given files and displays the lines which match the given pattern.

grep error *.logDisplays all the lines containing error in the *.log files

grep i error *.logSame, but case insensitive

grep ri error .Same, but recursively in all the files in . and its subdirectories

grep v info *.logOutputs all the lines in the files except those containing info.


The sort command

sort <file>Sorts the lines in the given file in character order and outputs them.

sort r <file>Same, but in reverse order.

sort ru <file>u: unique. Same, but just outputs identical lines once.

More possibilities described later!


The Unix and GNU / Linux command line

Standard I/O, redirections, pipes


Standard output

More about command output

All the commands outputting text on your terminal do it by writing to their standard output.

Standard output can be written (redirected) to a file using the > symbol

Standard output can be appended to an existing file using the >> symbol


Standard output redirection examples

ls ~saddam/* > ~gwb/weapons_mass_destruction.txt

cat obiwan_kenobi.txt > starwars_biographies.txtcat han_solo.txt >> starwars_biographies.txt

echo “README: No such file or directory” > READMEUseful way of creating a file without a text editor.Nice Unix joke too in this case.

cat obiwan_kenobi.txt >! starwars_biographies.txtWhen the file exists, the exclamation point can be used with redirection to indicate that the file should be overwritten.


Standard input

More about command input

Lots of commands, when not given input arguments, can take their input from standard input.

sortwindowslinux[Ctrl][D]linuxwindows

sort < participants.txtThe standard input of sort is taken from the given file.

sort takes its input fromthe standard input: in this case,what you type in the terminal(ended by [Ctrl][D])


Pipes

Unix pipes are very useful to redirect the standard output of a command to the standard input of another one.

Examples

cat *.log | grep i error | sort

grep ri error . | grep v “ignored” | sort u \ > serious_errors.log

cat /home/*/homework.txt | grep mark | more

This one of the most powerful features in Unix shells!


Special devices (1)

Device files with a special behavior or contents

/dev/nullThe data sink! Discards all data written to this file.Useful to get rid of unwanted output, typically log information:mplayer black_adder_4th.avi &> /dev/null

/dev/zeroReads from this file always return \0 charactersUseful to create a file filled with zeros:dd if=/dev/zero of=disk.img bs=1k count=2048

See man null or man zero for details



Task control


Full control on tasks

Since the beginning, Unix supports true preemptive multitasking.

Ability to run many tasks in parallel, and abort them even if they corrupt their own state and data.

Ability to choose which programs you run.

Ability to choose which input your programs takes, and where their output goes.


Processes

“Everything in Unix is a fileEverything in Unix that is not a file is a process”

Processes

Instances of a running programs

Several instances of the same program can run at the same time

Data associated to processes:Open files, allocated memory, stack, process id, parent, priority, state...


Running jobs in background

Same usage throughout all the shells

Useful

For command line jobs which output can be examined later, especially for time consuming ones.

To start graphical applications from the command line and then continue with the mouse.

Starting a task: add & at the end of your line:

find_prince_charming cute clever rich &


Background job control

jobsReturns the list of background jobs from the same shell

[1] Running ~/bin/find_meaning_of_life withoutgod &[2]+ Running make mistakes &

fgfg %<n>Puts the last / nth background job in foreground mode

Moving the current task in background mode:[Ctrl] Zbg

kill %<n>Aborts the nth job.


Job control example

> jobs[1] Running ~/bin/find_meaning_of_life withoutgod &[2]+ Running make mistakes &

> fgmake mistakes

> [Ctrl] Z[2]+ Stopped make mistakes

> bg[2]+ make mistakes &

> kill %1[1]+ Terminated ~/bin/find_meaning_of_life withoutgod


Listing all processes

... whatever shell, script or process they are started from

ps uxLists all the processes belonging to the current user

ps aux (Note: ps edf on System V systems)Lists all the processes running on the systemps aux | grep bart | grep bashUSER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMANDbart 3039 0.0 0.2 5916 1380 pts/2 S 14:35 0:00 /bin/bashbart 3134 0.0 0.2 5388 1380 pts/3 S 14:36 0:00 /bin/bashbart 3190 0.0 0.2 6368 1360 pts/4 S 14:37 0:00 /bin/bashbart 3416 0.0 0.0 0 0 pts/2 RW 15:07 0:00 [bash]

PID: Process idVSZ: Virtual process size (code + data + stack)RSS: Process resident size: number of KB currently in RAMTTY: TerminalSTAT: Status: R (Runnable), S (Sleep), W (paging), Z (Zombie)...


Live process activity

top Displays most important processes, sorted by cpu percentage

top 15:44:33 up 1:11, 5 users, load average: 0.98, 0.61, 0.59Tasks: 81 total, 5 running, 76 sleeping, 0 stopped, 0 zombieCpu(s): 92.7% us, 5.3% sy, 0.0% ni, 0.0% id, 1.7% wa, 0.3% hi, 0.0% siMem: 515344k total, 512384k used, 2960k free, 20464k buffersSwap: 1044184k total, 0k used, 1044184k free, 277660k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND3809 jdoe 25 0 6256 3932 1312 R 93.8 0.8 0:21.49 bunzip22769 root 16 0 157m 80m 90m R 2.7 16.0 5:21.01 X3006 jdoe 15 0 30928 15m 27m S 0.3 3.0 0:22.40 kdeinit3008 jdoe 16 0 5624 892 4468 S 0.3 0.2 0:06.59 autorun3034 jdoe 15 0 26764 12m 24m S 0.3 2.5 0:12.68 kscd3810 jdoe 16 0 2892 916 1620 R 0.3 0.2 0:00.06 top

You can change the sorting order by typingM: Memory usage, P: %CPU, T: Time.

You can kill a task by typing k and the process id.


Killing processes (1)

kill <pids>Sends an abort signal to the given processes. Lets processes save data and exit by themselves. Should be used first. Example:kill 3039 3134 3190 3416

kill 9 <pids>Sends an immediate termination signal. The system itself terminates the processes. Useful when a process is really stuck (doesn't answer to kill 1).

kill 9 1Kills all the processes of the current user. 1: means all processes.



MiscellaneousVarious commands


Compiling simple applications

The compiler used for all Linux systems is GCChttp://gcc.gnu.org

To compile a singlefile application, developed in C :gcc o test test.c

Will generate a test binary, from the test.c source file

For C++ :g++ o test test.cc

The Wall option enables more warnings

To compile sources files to object files and link the application :gcc c test1.cgcc c test2.cgcc o test test1.o test2.o

gcc automatically calls the linker ld


Remote Shells

ssh ele.uri.eduLogon to ele.uri.edu with a remote shell

ssh [email protected] to ele.uri.edu as user toolan with a remote shell

scp [email protected]:rfile lfile Copy rfile from ele.uri.edu to lfile on local system

scp r ldir [email protected]:/home/rdirCopy local directory rdir and all of its contents to remote system ele.uri.edu and put in /home/rdir

scp is just like cp, but the source and/or destination can be on a remote system

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